This invention relates to two-stroke combustion engines and more particularly to port-controlled two-stroke combustion engines wherein the movement of a piston within a cylinder acts to open and close intake, exhaust and transfer ports of that engine.
Port-controlled two-stroke combustion engines generally comprise a cylinder, a piston working in the cylinder, a crankcase and one or more flywheels rotatably mounted in the crankcase. An intake port and an exhaust port are provided in the cylinder wall respectively for admitting and exhausting combustible mixture to the engine. Transfer ports are also provided in the cylinder wall. Transfer passages, which each extend between a respective crankcase transfer port and a respective combustion chamber transfer port, convey air/fuel mixture from the crankcase to the combustion chamber.
In two-stroke engines of this type, the combustible mixture flows through the intake port into the cylinder at the crankcase side of the piston when the piston is adjacent the cylinder head. As the engine fires and the piston moves towards the crankcase, this combustible mixture is then compressed in the crankcase. When the piston approaches its extremity of travel closest to the crankcase, it uncovers transfer ports in the cylinder wall. This allows the combustible mixture which has been compressed in the crankcase to flow along the transfer passages into the cylinder between the piston and the cylinder head.
The power of such two-stroke engines is dependent upon the efficiency of the circulation of the combustible mixture when the engine is in operation and, in particular, the efficient transfer of combustible mixture from inside the crankcase to the combustion chamber. For this reason, conventional two-stroke engines generally attempt to maximize the total cross-sectional of the transfer passages and associated porting by providing at least one transfer passage on each side of the cylinder wall and an auxiliary transfer passage on the same side of the cylinder wall as the intake port.